This project will complete the development of a diode laser based system for continuous thermodilution measurements of cardiac output. Our system volumetrically heats a bolus of blood 2-4 degrees Celsius using 980 nm diode lasers irradiated through a diffuser. A thermistor located in the pulmonary artery measures the blood temperature. This system measures cardiac output continuously since it does not use saline bolus injections and therefore avoids fluid overload of the blood system. It is more accurate than systems that heat blood using resistive heaters because volumetric heating delivers significantly more energy than resistive heaters. which improves the signal-to-noise ratio of the temperature measurements. Sensitivity to respiration-induced thermal noise is reduced through adaptive filtering or through optional synchronization of the measurements to the respiration cycle. Safety is ensured by the real-time measurement of peak blood temperature, and by a system that continually checks the fiber- optic integrity. This system will therefore be capable of continuously measuring cardiac output with accuracy equivalent to bolus-based thermodilution measurements. The measurements will be automated, eliminating the need for operator intervention. The resulting system will be safe, reliable, and easy to use. The accuracy and safety of this system will be verified in a pig model. PROPOSED COMMERCIAL APPLICATIONS This project will develop a catheter and system that will be capable of measuring cardiac output accurately and continuously. It offers cardiologists and critical care facilities measurements that are not currently possible, and will automate those same measurements. It could dramatically increase the quality of care in these facilities.